1. Field of the Invention
The present invention relates to optical communication transmitters and, more particularly, to predistortion circuits used with optical communication transmitters.
2. Description of the Background Art
There are multiple laser/modulator configurations. A design goal of most of these configurations is to limit nonlinearities of the output photonic signal generated in response to the input signal. One embodiment of optical transmission systems include directly modulated lasers whose outputs are connected to optical amplifiers and single mode optical fiber links. The use of erbium-doped fiber amplifiers (EFDA's) in one embodiment of optical transmission system, i.e. AM VSB cable TV (cable TV or CATV) systems operating at, for example, a wavelength of 1.55 μm, is desirable because the EFDAs can provide high power output photonic signals. Splitters utilized in networks such as AM-VSB cable TV systems typically feed into a plurality of output fiber branches having variable lengths. Each output photonic signal is transmitted over optical fiber links to a distinct optical receiver. High power output photonic signals from the EFDAs have high splitting ratios and therefore can be split into multiple signals by the splitter, with certain different ones of the multiple signals following different fiber links.
Both lasers and modulators exhibit distortions that can result in signal degradation. Chirp is an undesired shifting and broadening, i.e., a distortion, of the output spectrum of a laser at relatively high modulation frequencies.
Unfortunately, the chirp produced by directly modulated DFB lasers leads to signal distortion in fiber with chromatic dispersion at the operating wavelength and in EDFAs having a gain that varies with wavelength. Thus, AM-VSB cable TV systems require either dispersion compensation or external modulation to compensate for the signal distortion.
In dispersion compensation for directly compensated lasers, a dispersion-compensating fiber is installed in each existing fiber branch to compensate for the distortion. Each dispersion compensation fiber is configured to compensate for the different splitter output branch lengths. Dispersion compensation is labor-intensive, cumbersome, costly, and complicates the construction and/or expansion of networks.
Another laser/modulator configuration includes external modulation lasers in which the output of the laser is coupled to the input of a distinct externally modulated source. The output of the externally modulated source connects to the existing fiber. Externally modulated sources produce a low chirp, thereby obviating the need for dispersion compensation. A LiNbO3 Mach-Zender modulator is a commercially available externally modulated source, but that is relatively expensive.
Electroabsorption (EA) modulators are another form of external modulation in which the modulator is integrated onto the same wafer as the laser. Electroabsorption modulated lasers (EMLs) are a type of EA modulator/laser in which the laser and abutted modulator are integrated into the same element by being grown on the same wafer. EMLs also produce a low chirp, are compact, and are potentially less expensive than other externally modulated sources.
A nonlinear transfer function (output power vs. input bias voltage) of a modulator typically has an inflection point. The inflection point is represented by where the second derivative of the curve is zero. Biasing the modulator at the inflection point of its transfer function minimizes composite second-order (CSO) distortions. In some devices, this relatively low CSO is still unacceptably high. The nonlinearity of the transfer function, parameterized by the composite-triple-beat (CTB) that represents the third-order distortion is high at the bias point that minimizes the CSO.
Various linearization techniques have been proposed for modulators in optical transmission systems. An EA modulator, for example, displays a substantially linear light vs. current transfer function at lower frequencies since the photocurrent produced is nearly proportional to the light absorbed at frequencies up to about 10 Mhz. However, providing a linearization range in EA modulators of about 600 Mhz, such as could be utilized in high-frequency optical networks is difficult at these frequencies.
Feedforward is another linearization technique that is described in U.S. Pat. No. 6,072,616 that issued on Jun. 6, 2000 to Cohen et al. and entitled “APPARATUS AND METHODS FOR IMPROVING LINEARITY AND NOISE PERFORMANCE OF AN OPTICAL SOURCE” (incorporated herein by reference). Feedforward has not yet been demonstrated using EA modulators. Feedforward relates generally to a process of supplying an input signal generated by an optical source, and supplying the input signal to a receiver for use in correcting a signal received from the optical source. Feedforward typically requires the use of a second laser at the transmitter. The linearization effectiveness of feedforward is limited to those instances where the wavelength of the primary transmitting laser match the wavelength of the second modulating laser. Where the wavelengths of the two lasers differ, the two signals will arrive at the receiver having undergone different delays.
Predistortion is presently the predominant technique for linearizing directly modulated lasers and Mach-Zehnder modulators and it is also regarded as being the most promising technique for linearizing EA modulators. The article by M. Nazarathy et al. entitled “Progress in Externally Modulated AM CATV Transmission Systems”, J. Lightwave Technology., vol. 11, pp. 82-104 (1993) describes a correction technique for CTE distortion in which a linearizer signal is combined into the signal upstream of an electro-optic modulator to provide a more linear output photonic signal from the modulator. In this article, the modulator and linearizer transfer characteristics are modeled as an odd-function power series expansion. A distortion limiting of up to the third-order term of the expansion is reported. The Nazarathy et al. reference does not disclose a technique by which fifth-order predistortion intermodulation distortion (IMD) or even-order predistortion IMD, may be independently controlled.
Some predistortion circuits have used separate signal paths to generate even-order and odd-order predistortion IMD. For example, a first signal path of the predistortion circuit contains signal components that limit second-order modulator IMD. A second signal path of the predistortion circuit contains signal components that limit third-order modulator IMD. A third signal path of the predistortion circuit contains signal components that limit fourth-order modulator IMD. The first, second, and third signal paths have to be connected with their outputs each being input into the modulator. More complex and expensive splitting, timing (i.e. delay), and combining devices are required for predistortion circuits including multiple signal paths compared to predistortion circuits with a single signal path. The predistortion circuits relying on separate signal paths result in greater cost, additional components that can fail, and reduced performance (due to the added signal-strength attenuation and frequency response imperfections).
Therefore, a need exists in the art for a predistortion circuit that can compensate for even-order and odd-order IMD.